1. FIELD OF THE INVENTION
This invention relates to a microwave frequency channel combining apparatus, commonly referred to as a multiplexer and to a method of constructing such a multiplexer. In particular, this invention relates to a modular contiguous channel multiplexer that provides complete flexibility in that any number of channels can be contiguously combined in any preferred order of relative frequencies and any number of channels can be added at any future time without affecting the performance of the existing channels in the multiplexer. In addition, any number of channels can be removed from an upstream position in a multiplexer relative to the remaining channels without affecting the performance of other channels in the multiplexer.
2. DESCRIPTION OF THE PRIOR ART
It is common for modern multichannel commercial communication spacecraft ground stations to combine a plurality of signals before the signal is transmitted to the spacecraft. One manner of combining such a plurality of signals is to use cascaded directional filters to form a non-contiguous multiplexing network. The design and properties of directional filters have been known for many years. In proceedings of the I.R.E., Volume 44, Number 8, August 1956, Pages 1018-1024, by S. B. Cohn and F. S. Coale, the use and design of directional channel separation filters is discussed. Where enough guard band exists between adjacent channels of this type of arrangement, no interaction will exist between the filters of adjacent channels when the channels are cascaded together. Consequently, each channel is a separate module which is able to be added or removed from the main chain without affecting the existing or remaining multiplexer. Unfortunately, when a multiplexer has an arrangment of contiguous channels relative to their operating frequencies, the guard bands between adjacent channels are insufficient or non-existent and strong interchannel interactions result. To solve this problem, it is common to construct two non-contiguous multiplexers adjacent to one another and to combine them using a 3 dB hybrid. Even though this arrangement results in a power loss of approximately fifty percent and has other requirements that make it extremely expensive to construct and to operate, these types of multiplexers are still widely used in ground stations on a worldwide basis. One reason for this widespread use is that this type of multiplexer allows total flexibility in adding channels at any time without any sacrifice in the performance of other channels.
In U.S. Pat. No. 4,029,902, issued on June 14th, 1977 to Bell, Jr. et al., entitled, "Contiguous Channel Multiplexer", an improved multiplexer is described whereby one or both of the preselected bandpass characteristics is extended past their associated band edges and into the reflective loss regions of the adjacent channel or channels. All channel filters use the same symmetrical filter design. Unfortunately, the multiplexer described in the Bell patent suffers from three disadvantages. Firstly, channels of the multiplexer must be arranged in either ascending or descending order of channel frequency. This does not constitute a problem for multiplexers used in spacecraft but is a very important limitation for multiplexers used for ground stations. Secondly, the multiplexing method causes asymmetry in the channel performance. With the present extensive use of a digital signal, for example, PSK or QPSK, channel asymmetry can reduce data rate and consequently can reduce revenue. Thirdly, channels cannot be added to or removed from existing multiplexer as asymmetries would then be introduced into the performance of adjacent channels, hence reducing their revenue earning capacity. However, the multiplexer described in the Bell patent does overcome the problem of the fifty percent power loss and the disadvantages generally apply only to the use of the multiplexer in a ground station.
In U.S. Pat. No. 4,258,435 issued to Ralph Levy et al. entitled "Manifold Multiplexer", there is described a multiplexer that provides minimum loss and minimum sizes, which are critical to spacecraft application. Unfortunately, the multiplexer described in the Levy patent is inflexible and relatively expensive, when compared to the present invention. The multiplexer described in the Levy patent is optimized and tuned as a complete unit and any change in the operating frequency and/or band width of a single channel will require a complete redesign of the entire multiplexer. This limitation is highly undesirable for multiplexers used in ground stations. The multiplexer described in the Levy patent also overcomes the problem of the fifty percent power loss.
With multiplexers used in spacecraft, the fact that channels cannot be added or removed from the multiplexer without redesigning the entire multiplexer, is not a serious limitation as changes cannot be made in the design once the spacecraft is launched in any event. Also, when a multiplexer is designed for use in a spacecraft, the number and relative frequencies of channels is known and there is no difficulty in designing the multiplexer so that the channels are arranged in either ascending or descending order of frequency. However, in multiplexers used in ground stations, a ground station owner may start with only two or three channels, for example, channels 1, 2 and 5. Subsequently, the ground station operator might be assigned a new channel, for example, channel 4, where consecutive channel numbers represent the operating frequency of each channel. The operator could be assigned this additional channel 4 because of growth achieved within the industry or because one of the existing channels is no longer available from the spacecraft and the new channel replaces the existing channel. With the multiplexers known in the prior art, channel 4 cannot be added to the arrangement of channels 1, 2 and 5 without redesigning and reconstructing the entire multiplexer. Of course, it may be necessary to add or remove additional channels on more than one occasion. If the multiplexer must be entirely redesigned each time, the cost of adding or removing channels to existing multiplexers can be prohibitive. Multiplexers used in ground stations are sometimes referred to as combiners. Ground stations usually do not have all of the channels that a satellite has but only a few nonconsecutive channels.